Aeolian Islands, Italy)

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Earth-prints Repository ANNALS OF GEOPHYSICS, VOL. 48, N. 6, December 2005 The high resolution bathymetric map of the exhalative area of Panarea (Aeolian Islands, Italy) Marco Anzidei (1), Alessandra Esposito (1), Giovanni Bortoluzzi (2) and Francesco De Giosa (3) (1) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (2) Istituto di Geologia Marina (ISMAR), CNR, Bologna, Italy (3) Coastal Consulting and Exploration srl, Bari, Italy Abstract On November 3, 2002 a shallow submarine gas eruption occurred in an area of 2.3 km2 east of Panarea (Aeo- lian volcanic arc, Southern Thyrrenian Sea, Italy). The exhalative area, surrounded by the islets of Dattilo, Panarelli, Lisca Bianca, Bottaro and Lisca Nera, has been known since historical times for the hydrothermal activity related to the Panarea volcanic complex. Due to the exceptional characteristics of the phenomenon, differ- ent geological, geochemical, geophysical and studies were carried out in this still poorly known volcanic area. A particular effort was devoted to producing a high resolution bathymetric map that also aimed to estimate the amount and location of the active exhalative centers and their variations in space and time. Data were obtained by three RTK multibeam surveys performed between December 2002 and December 2003. Here we show and discuss the technical details of the bathymetric surveys, the bathymetric map at 0.5 m resolution, and the accurate location of the 606 main exhalative centres active during the 2002-2003 crisis. The bathymetric data and the maps show two prevailing principal NE-SW and NW-SE alignments that match the spatial distribution of the exhalation centres. The accurate positioning at submeter accuracy of the gas vents is useful in the monitoring activity and to study their temporal and spatial variability. Key words Panarea – bathimetry – DEM – hy- to a slab deepening under the Calabrian Arc drotermal systems (Chiarabba et al., 2005), active crustal defor- mation (Anzidei et al., 2000; Hollenstein et al., 2003; Tallarico et al., 2003; D’Agostino and 1. Introduction Selvaggi, 2004; Serpelloni et al., 2005), active volcanoes (Lipari, Vulcano and Stromboli) and Panarea Island is located in the Southern hydrothermalism. The above features witness a Tyrrhenian Sea and is part of the volcanic arc of long lasting activity, as described by Italiano the Aeolian Islands that is made up of seven is- and Nuccio (1991), Gamberi et al. (1997), lands, some minor islets and several seamounts Calanchi et al. (1999, 2002), De Astis et al. (fig. 1a). The area is characterized by a com- (2003) and Caliro et al. (2004). plex geodynamics (Falsaperla et al., 1999) be- Panarea is the smallest among the Aeolian ing affected by shallow to deep seismicity due Islands, representing the subaerial portion of a submarine stratovolcano about 2000 m high and 20 km wide (Gabbianelli et al., 1993; Gam- beri et al., 1997). East of Panarea, the islets of Basiluzzo, Dattilo, Panarelli, Lisca Bianca, Mailing address: Dr. Marco Anzidei, Istituto Naziona- le di Geofisica e Vulcanologia, Via di Vigna Murata 605, Bottaro, Lisca Nera and Le Formiche (hereafter 00143 Roma, Italy, e-mail: [email protected] referred as the ‘Islets’) form an archipelago that 899 Marco Anzidei, Alessandra Esposito, Giovanni Bortoluzzi and Francesco De Giosa a b c Fig. 1a-c. a) Regional setting of Panarea Island; b) Panarea Archipelago; c) the ALSEA vessel used during bathymetric surveys. In the map are also displayed the locations of the tide gauge, the TyrGeoNet GPS station and the area investigated by the bathymetric surveys. emerges from the eastern flank of the volcano air, death of fishes, mud emerging from seafloor (fig. 1b). Panarea is made up of dacite to rhyo- occurred on 126 b.C. This event is likely referred lite lava domes, dated up to 149±5 kyr to the formation of Vulcanello, nowadays linked (Calanchi et al., 1999), interbedded with pyro- to Vulcano Island. The exhalative activity at clastic deposits. The island is topped by the py- Panarea has been described by Houel (1782), roclastic deposits of Punta del Torrione forma- Dolomieu (1783), Ferrara (1810), Spallanzani tion, dated between 42 kyr and 11 kyr, which (1825), Dumas (1860), Mercalli (1883), Luigi also outcrop at Salina, Lipari, Filicudi, Vulcano Salvatore d’Austria (1895), Romano (1973), Ital- and along the northern coast of Calabria and iano and Nuccio (1991), Gabbianelli et al. Sicily (Lucchi et al., 2003). (1990), Calanchi et al. (1995), Anzidei (2000), Boiling of the sea-water has been described Caliro et al., (2004), Caracausi et al. (2005). since historical times in the Aeolian Islands by At the beginning of November 2002, during Titus Livius, Strabo, Julis Obsequens, Orosius the eruptions of Etna and Stromboli volcanoes, Palulus, Posidonius and Plinius the Elder (SGA, a submarine gas eruption started in a shallow 1993). An event that produced sudden changes in area up to 30 m water depth and 2.3 km2 of sur- the sea level, hot steam from sea surface, stinky face, bordered by the Islets. The event occurred 900 The high resolution bathymetric map of the exhalative area of Panarea (Aeolian Islands, Italy) suddenly, without significant seismicity (Sac- in the early days of December 2002, with par- carotti et al., 2003) and reached an intensity tial repetitions in selected areas during July level never observed before during the last cen- 2003 and December 2003. tury (SGA, 1996). The gas output was estimat- ed to be 109 l/day, two orders of magnitude higher than that previously measured (Caliro et 2. Bathymetric surveys al., 2004). The most active exhalation center (EC-1, fig. 7b) was located close to the SW A multibeam high resolution survey that wall of the islet of Bottaro, with gas flowing covered an area of about 9 km2 around the Islets vigorously up to the water surface. To provide was performed for the first time in December 8- the present day locations of the submarine gas 12, 2002, using the Alsea boat of Coastal Con- Exhalation Centres (EC) with respect to previ- sulting and Exploration Company (fig. 1c), ous studies (Gabbianelli et al., 1990; Anzidei, equipped with an ultra high resolution Reson 2000) and a first high resolution morphobathy- Seabat 8125 multibeam (240 beams, 120° sec- metric map of the exhalation centres, bathymet- tor coverage, 455 kHz) (table I). ric surveys were planned and performed during Before starting the surveys, a check of the the maximum activity. Surveys were carried out health of the GPS/RTK data link was performed Table I. Instrumentation used during the bathymetric surveys. Instrumentation Type Features Vessel M/B ALSEA - Differential GPS receiver Ashtech-Aquarius 02 10 mm+0.5 ppm, XY (RTK mode) 20 mm+1.0 ppm, Z Multi beam Reson SeaBat 8125 Frequency 455 kHz Angle 120° Beams 240 Max depth 120 m Resolution 6 mm Gyrocompass SG Brown Meridian 0.05° static secant latitude 0.2° dynamic secant latitude Velocity sounder Navitronic SVP 15 Resolution 0.1 m/s Accuracy ±0.25 m/s CTD Microcat SBE 37-SI Conductivity accuracy 0.0003 S/m Temperature accuracy 0.002°C Motion sensor TSS DMS 2-05 Waves Static accuracy 5 cm Dynamic accuracy 0÷20 s period Pitch and roll Static accuracy 0.025° Dynamic accuracy 0.03°±5° Digital tide gauge Ott Hydrometry Accuracy <1 cm (at Panarea) Orphimedes On board computers Pentium IV- Data acquisition Windows 2000 Software RESON B.V. PDS 2000 Navigation and data analysis Software CARIS-HIPS 5.2 Data analysis Software ArcView GIS 8.2 Data management 901 Marco Anzidei, Alessandra Esposito, Giovanni Bortoluzzi and Francesco De Giosa Fig. 2. Sketch (not in scale) of the Panarea Pier with the location of the tide gauge and the geodetic benchmark used to define its coordinates with respect to the TyrGeoNet GPS station of Panarea. Table II. Coordinates of PANA TyrGeoNet station and tide gauge benchmark (WGS84, ellipsoidal heights). Station Geographic UTM Height (m) PANA (TyrGeoNet) Lat N 38°37l58.909 N 4276057.468 144554 Long E 15°04l26.457 E 506442.1583 Tide Gauge Lat N 38°38l16.772 N 4276608.335 43.30 Long E 15°04l40.257 E 506775.313 throughout the planned survey, having previ- urements using a Total Station, with reference ously found a local shadow area behind the to the GPS geodetic monument of PANA, locat- eastern wall of Dattilo. The bathymetric datum ed at Panarea (fig. 1a), which belongs to the was established by measuring the sea level Tyrrhenian Geodetic Network (TyrGeoNet), through a high accuracy tide gauge, temporari- whose 3D coordinates are known at a few mm ly installed along the pier at S. Pietro, Panarea level (Anzidei et al., 1995; Serpelloni et al., (fig. 1a and 2). The instrumental height of the 2005), (table II). zero level was referred to a WGS84 geodetic The decimetric positioning of the vessel benchmark previously set up over the pier (fig. was computed by GPS technique in RTK mode. 2 and table II). The latter was measured by ge- Real time coordinates were obtained by in- odetic space techniques using a couple of dual stalling an Ashtech Aquarius reference station frequency GPS receivers and by ground meas- located on the GPS station PANA and transmit- 902 The high resolution bathymetric map of the exhalative area of Panarea (Aeolian Islands, Italy) ting the differential corrections by a High Fre- quency link at 1 Hz rate to the mobile Aquarius receiver which was placed on board of the vessel. In addition to this, a Sg-Brown Meridian Surveyor gyrocompass, a Tss DM505 MRU and a Fugro Omnistar Differential GPS, were interfaced to the Reson PDS2000 Navigation software (www.reson.com) for data acquisition (multibeam and positioning), control, calibra- tion and pre-processing.

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